1. Field of the Invention
The invention relates generally to bearing assemblies for mud motors used in drilling of oil, gas, and water wells. More particularly, the invention relates to pressure compensation systems for oil-sealed bearing assemblies.
2. Background of the Technology
In drilling a wellbore into the earth, such as for the recovery of hydrocarbons or minerals from a subsurface formation, it is conventional practice to connect a drill bit onto the lower end of an assembly of drill pipe sections connected end-to-end (commonly referred to as a “drill string”), and then rotate the drill string so that the drill bit progresses downward into the earth to create the desired wellbore. In conventional vertical wellbore drilling operations, the drill string and bit are rotated by means of either a “rotary table” or a “top drive” associated with a drilling rig erected at the ground surface over the wellbore (or, in offshore drilling operations, on a seabed-supported drilling platform or a suitably adapted floating vessel).
During the drilling process, a drilling fluid (also commonly referred to in the industry as “drilling mud”, or simply “mud”) is pumped under pressure downward from the surface through the drill string, out the drill bit into the wellbore, and then upward back to the surface through the annular space between the drill string and the wellbore. The drilling fluid, which may be water-based or oil-based, is typically viscous to enhance its ability to carry wellbore cuttings to the surface. The drilling fluid can perform various other valuable functions, including enhancement of drill bit performance (e.g., by ejection of fluid under pressure through ports in the drill bit, creating mud jets that blast into and weaken the underlying formation in advance of the drill bit), drill bit cooling, and formation of a protective cake on the wellbore wall (to stabilize and seal the wellbore wall).
Particularly since the mid-1980s, it has become increasingly common and desirable in the oil and gas industry to use “directional drilling” techniques to drill horizontal and other non-vertical wellbores, to facilitate more efficient access to and production from larger regions of subsurface hydrocarbon-bearing formations than would be possible using only vertical wellbores. In directional drilling, specialized drill string components and “bottomhole assemblies” (BHAs) are used to induce, monitor, and control deviations in the path of the drill bit, so as to produce a wellbore of desired non-vertical configuration.
Directional drilling is typically carried out using a “downhole motor” (alternatively referred to as a “mud motor”) incorporated into the drill string immediately above the drill bit. A typical mud motor includes several primary components, as follows (in order, starting from the top of the motor assembly):                a top sub adapted to facilitate connection to the lower end of a drill string (“sub” being the common general term in the oil and gas industry for any small or secondary drill string component);        a power section comprising a positive displacement motor of well-known type, with a helically-vaned rotor eccentrically rotatable within a stator section;        a drive shaft enclosed within a drive shaft housing, with the upper end of the drive shaft being operably connected to the rotor of the power section; and        a bearing section comprising a cylindrical mandrel coaxially and rotatably disposed within a cylindrical housing, with an upper end coupled to the lower end of the drive shaft, and a lower end adapted for connection to a drill bit.The mandrel is rotated by the drive shaft, which rotates in response to the flow of drilling fluid under pressure through the power section. The mandrel rotates relative to the cylindrical housing, which is connected to the drill string.        
In drilling processes using a mud motor, drilling fluid is circulated under pressure through the drill string and back up to the surface as in conventional drilling methods. However, the pressurized drilling fluid exiting the lower end of the drill pipe is diverted through the power section of the mud motor to generate power to rotate the drill bit.
The bearing section must permit relative rotation between the mandrel and the housing, while also transferring axial thrust loads between the mandrel and the housing. Axial thrust loads arise in two drilling operational modes: “on-bottom” loading, and “off-bottom” loading. On-bottom loading corresponds to the operational mode during which the drill bit is boring into a subsurface formation under vertical load from the weight of the drill string, which in turn is in compression; in other words, the drill bit is on the bottom of the wellbore. Off-bottom loading corresponds to operational modes during which the drill bit is raised off the bottom of the wellbore and the drill string is in tension (i.e., when the bit is off the bottom of the wellbore and is hanging from the drill string, such as when the drill string is being “tripped” out of the wellbore, or when the wellbore is being reamed in the uphole direction). This condition occurs, for instance, when the drill string is being pulled out of the wellbore, putting the drill string into tension due to the weight of drill string components. Tension loads across the bearing section housing and mandrel are also induced when circulating drilling fluid with the drill bit off bottom, due to the pressure drop across the drill bit and bearing assembly
Accordingly, the bearing section of a mud motor must be capable of withstanding thrust loads in both axial directions, with the mandrel rotating inside the housing. A mud motor bearing section may be configured with one or more bearings that resist on-bottom thrust loads only, with another one or more bearings that resist off-bottom thrust loads only. Alternatively, one or more bi-directional thrust bearings may be used to resist both on-bottom and off-bottom loads. A typical thrust bearing assembly comprises bearings (usually but not necessarily roller bearings contained within a bearing cage) disposed within an annular bearing containment chamber.
Bearings contained in the bearing section of a mud motor may be either oil-lubricated or mud-lubricated. In an oil-sealed bearing assembly, the bearings are located within an oil-filled reservoir in an annular region between the mandrel and the housing, with the reservoir being defined by the inner surfaces of the housing and the outer surface of the mandrel, and by sealing elements at each end of the reservoir. Because of the relative rotation between the mandrel and the housing, these sealing elements must include rotary seals.
Mud motor bearing sections also include multiple radial bearings to maintain coaxial alignment between the mandrel and the bearing housing. In an oil-sealed assembly, the radial bearings can be provided in the form of bushings disposed in an annular space between the inner surface of the housing and the outer surface of the mandrel. It is desirable to maximize radial support for the mandrel in order to maximize the mandrel's resistance to flexural stresses induced when drilling non-straight wellbores.
An oil-sealed bearing assembly must incorporate pressure compensation means, whereby the volume of the annular oil reservoir is automatically adjusted to compensate for changes in oil volume due to temperature changes. In addition, certain types of elastomeric rotary seals (such as KALSI SEALS®) are designed to slowly pump oil underneath the seal interface, and this causes a gradual reduction in oil volume which also must be compensated for. For optimum performance of the rotary seal, it is ideal for the sealing surface of the mandrel to be as wear-resistant as possible, while having a very fine surface finish.
A common method of providing pressure compensation in an oil-sealed bearing assembly uses an annularly-configured piston disposed within an annular region (or “piston chamber”) between the housing and mandrel. The outer diameter (O.D.) of the piston is sealed against the inner bore of the housing (by means of one or more sliding seals, such as O-rings), and also may incorporate anti-rotation seals to ensure that the piston does not rotate relative to the housing. The inner diameter (I.D.) of the piston is sealed against the mandrel by means of a rotary seal, which rotates relative to the mandrel during operation, and also slides axially along the mandrel as the piston moves. The rotary seal and sliding seals associated with the piston thus define the upper end of the oil reservoir within the bearing assembly.
A sufficient length of the mandrel below the piston's initial position must remain uninterrupted to accommodate the piston travel that will occur as oil volume varies over time (whether due to temperature change or oil loss). The housing bore must be similarly uninterrupted along this length, forming a cylindrical oil reservoir. The uppermost radial support is thus located at a point below the oil reservoir. Therefore, a significant length of the mandrel in a conventional oil-sealed mud motor bearing section is not radially supported.
Alternatively, radial support for the mandrel may be provided to some extent by the pressure-compensating piston itself. However, the length of radial support is limited to the length of the piston (which desirably should be minimized), and the mandrel will still be unsupported along the length of the oil reservoir (said length of which will be greatest when the oil reservoir is full and the piston is at its uppermost position).
For optimum performance of the rotary seal, it is ideal for the sealing surface of the mandrel to be as wear-resistant as possible, with a very fine surface finish. This is typically provided through the use of a surface treatment such as an abrasion-resistant, diamond-ground coating. To accommodate axial translation of the piston within the piston chamber, the surface treatment of the mandrel needs to be provided over a length corresponding to at least the range of travel of the piston's rotary seal, and preferably the full length of the piston chamber.
Accordingly, there remains a need in the art for a pressure compensation system for oil-sealed mud motor bearing assemblies that provides radial support for the portion of the mandrel corresponding to the stroke of the pressure-compensating piston. Embodiments disclosed herein are directed to these needs.